How to verify voltage for sensor

1. Locate plug CN40 on the main board and remove it. Measure dc voltage with the plug disconnected on the plug socket where you removed the CN40 plug. the voltage should be 4.9-5 VDC between the pins.  
2.  Verify 24 vac voltage is present on CN16. If voltage is correct, proceed to step 3. If voltage is below 19 VAC, unplug CN16 and check voltage from transformer on the plug removed. If 24 VAC is present, remove any field installed low voltage wiring to the unit control board to locate voltage drop. If voltage is still below 19 VAC after removing all field wiring, disconnect all other sensors to see if voltage returns to normal. If not, then replace main board. 
3. If voltages are correct to the board and there is no voltage an CN40, Replace board.

How to verify sensor operation and calibration

1. Locate the CN40 plug on the main board and remove it from the board for testing. 
2. Ohm the two wires in the plug that was removed from CN40 and check it against the OHM / Temperature chart below in Figure 2. You will need to also measure the actual temperature at the sensor to compare the ohm reading to temperature. See Figure 1 below.

Figure 1 

3. If the ohms do not match, if it is open or shorted to ground then check again at the actual sensor plug location with the sensor unplugged from the harness. If it is still not matching the chart ohms, open or shorted to ground replace the sensor. If when unplugged the sensor does measure correctly, replace harness to sensor.

4. If the harness and sensor ohm correctly and match the chart in figure 2, replace the main board. 

Figure 2 

Chart for sensors:

• T1

• T2

• T3

• T4 

WARNING:

Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.